Present day commercial lightwave systems use optical fibers to carry large amounts of multiplexed information over long distances from a transmit terminal to a receive terminal. Most long-haul transmission lines and a substantial portion of short-haul transmission lines such as inter-office and intra-office links, local area networks (LAN) and metropolitan area networks (MAN) are optical and, therefore, the information is carried over an optical fiber. A major advantage of transmitting information in optical form is the very large bandwidth and low losses associated with single mode optical fiber.
In a communication network, it is normally essential that signals from many transmission lines be cross-connected or switched to other transmission lines to provide flexibility and to permit traffic from one transmission line to be rerouted to diverse destinations. More specifically, switching is required in a network to provide A) provisioning; B) restoration in case of line failure; C) network reconfiguration; D) maintenance; E) operation; F) service, and the like.
At the present time, cross-connect switches used with optical transmission systems are either manual or electronic. In manual cross-connect switches the ends of the optical fibers are physically disconnected and reconnected to form new transmission paths. Electronic cross-connect switches which can connect up to 2048 input ports to 2048 output ports (2048.times.2048) are available. Unfortunately, however, using electronic cross-connect switches with optical networks has two major disadvantages. One is that the optical signal must be converted to electrical form prior to being coupled to the cross-connect switch; and converted back to optical form after leaving the output port of the cross-connect switch. The second disadvantage is that the electronic cross-connect switch can only operate at a data rate which is substantially less than that of the optical transmission line. The maximum data rate of a presently available electronic cross-connect switch is from 45 Mb/s to 155 Mb/s, whereas the data rate of an optical transmission line is about 2.5 Gb/s. Thus, because of this difference, the typical optical data signal carried by an optical transmission line must be converted to electrical form and then demultiplexed to its tributaries prior to being applied to the input ports of an electronic cross-connect switch.
Clearly, an improved cross-connect switch which can switch optical signals, in optical form, from one optical transmission line to another optical transmission line is required.